1. Field of the Invention
This invention relates generally to signal peak detection in data signal channels and, more specifically, to a digital peak-tracking circuit for a magnetic data storage read channel.
2. Discussion of the Related Art
In a magnetic data storage apparatus, the read channel converts the voltage waveform of a magnetic head output signal to a square waveform representing the original data signal. The head output signal is amplified, equalized and peak-detected to produce a digital signal for processing through a data separator. Because the head output signal contains both data and clock information, shifts of the signal peaks from their nominal timing position directly hinders the data separation procedure and increases the data error rate. The equalizer is a critical element for minimizing the effects of signal peak shifts in the read channel.
Equalization involves the addition of linear filters to modify the channel transfer function so as to provide more reliable data detection by compensating for some of the channel shortcomings. Most of the linear intersymbol interference in a computer tape recording system arises from the limited bandwidth of the analog channel and the roll-off of signal amplitude with increasing storage density. The equalization is designed to provide the best possible signal to the data separator. The read channel output signal depends on both the head output signal into the channel and the channel transfer function, which includes the analog channel plus all equalizing filters. The ideal equalizer produces pulses with well-defined peaks at times equivalent to the flux-reversal times in the recording medium.
One of the problems associated with optimal channel equalization is the problem of tracking an analog input signal amplitude so that the signal peaks can be detected independent of fluctuations in magnitude. This problem is normally solved by using analog circuitry designed to balance two conflicting requirements. The first requirement is to minimize signal ripple during normal operation and the second requirement is to increase tracking response time so that the peak amplitude tracking circuit can follow a quick signal drop. The usual capacitor-based analog circuitry known in the art must compromise ripple and response time and the resulting performance causes the peak tracking threshold output to droop between signal peaks, effectively increasing ripple.
There is a clearly-felt need in the art for a peak-threshold tracking circuit that is sensitive enough to track rapid fluctuations in the input signal without increasing signal ripple. That is, there is a need for a method that rapidly adjusts a peak-detection threshold level while also avoiding any "droop" in the threshold level between peaks in the signal. The related unresolved problems and deficiencies are clearly-felt in the art and are solved by this invention in the manner described below.